It is well-known in the art to use electric motors to provide power assisted steering in motor vehicles. Typically, these power steering systems include a conventional multi-axis gear reduction box between the motor and the steering shaft to reduce the torque applied by the motor to the shaft.
When these systems are functioning properly, the motor, in response to the signals from a control circuit, operates to assist the driver of the vehicle by turning the steering shaft via the gear reduction box in the same direction and at the same speed at which the driver turns the steering wheel. However, power assisted steering systems occasionally malfunction. As a result, the motor may either cease to operate entirely or it may operate eratically or continuously, because of malfunctions of the control circuit. When the motor fails to operate entirely, the driver must steer the vehicle without power assistance and, in addition, part of the effort required to turn the steering wheel is absorbed by the motor inertia since, even though it is idle, it remains coupled to the steering shaft.
The second type of failure can be more troublesome. Since the power assist motor may be more powerful than the driver, when the motor fails to respond properly to the driver's commands, the possibility exists that the motor will control the operation of the steered wheels regardless of the driver's actions. In these instances, the driver may lose steering control over the vehicle.
A partial solution to these problems is described in an application entitled "Fail-Safe Mechanism For An Electrical Power Assisted Steering System", filed on the same date and assigned to the assignee as the present invention. Briefly, that application discloses the use of a clutch mechanism used in conjunction with a DC electric motor and a multi-axis planetary gear reduction unit which connects the motor to the steering shaft of the vehicle. The clutch mechanism disclosed in that application is biased radially away from the ring gear of the gear reduction unit and is mechanically grounded to the housing of the unit. A control circuit, which controls the operation of the motor, monitors the operational state of the system and detects malfunctions. When the system is operating properly, the clutch is moved into fricitional contact with the ring gear to hold the ring gear stationary, thereby transmitting torque from the motor, through the reduction unit, to the steering shaft and providing power assisted steering. When the system malfunctions, the clutch is disengaged allowing the reduction unit to turn independently of motor, thus isolating the motor from the steering shaft.
This solution to the problem, however, has certain drawbacks. For example, the diameter of the gear reduction unit cannot be minimized because the clutch is mounted radially out from the ring gear of the gear reduction unit. Additionally, it is often desirable to add a torque limiter so that excessively high torque from the motor does not reach the steering shaft and vice versa. Including such a limiter in the above described fail-safe system may be difficult and would require installation of additional apparatus between the motor and the reduction unit, increasing the cost of the system.